Root/Documentation/filesystems/romfs.txt

1ROMFS - ROM FILE SYSTEM
2
3This is a quite dumb, read only filesystem, mainly for initial RAM
4disks of installation disks. It has grown up by the need of having
5modules linked at boot time. Using this filesystem, you get a very
6similar feature, and even the possibility of a small kernel, with a
7file system which doesn't take up useful memory from the router
8functions in the basement of your office.
9
10For comparison, both the older minix and xiafs (the latter is now
11defunct) filesystems, compiled as module need more than 20000 bytes,
12while romfs is less than a page, about 4000 bytes (assuming i586
13code). Under the same conditions, the msdos filesystem would need
14about 30K (and does not support device nodes or symlinks), while the
15nfs module with nfsroot is about 57K. Furthermore, as a bit unfair
16comparison, an actual rescue disk used up 3202 blocks with ext2, while
17with romfs, it needed 3079 blocks.
18
19To create such a file system, you'll need a user program named
20genromfs. It is available on http://romfs.sourceforge.net/
21
22As the name suggests, romfs could be also used (space-efficiently) on
23various read-only media, like (E)EPROM disks if someone will have the
24motivation.. :)
25
26However, the main purpose of romfs is to have a very small kernel,
27which has only this filesystem linked in, and then can load any module
28later, with the current module utilities. It can also be used to run
29some program to decide if you need SCSI devices, and even IDE or
30floppy drives can be loaded later if you use the "initrd"--initial
31RAM disk--feature of the kernel. This would not be really news
32flash, but with romfs, you can even spare off your ext2 or minix or
33maybe even affs filesystem until you really know that you need it.
34
35For example, a distribution boot disk can contain only the cd disk
36drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem
37module. The kernel can be small enough, since it doesn't have other
38filesystems, like the quite large ext2fs module, which can then be
39loaded off the CD at a later stage of the installation. Another use
40would be for a recovery disk, when you are reinstalling a workstation
41from the network, and you will have all the tools/modules available
42from a nearby server, so you don't want to carry two disks for this
43purpose, just because it won't fit into ext2.
44
45romfs operates on block devices as you can expect, and the underlying
46structure is very simple. Every accessible structure begins on 16
47byte boundaries for fast access. The minimum space a file will take
48is 32 bytes (this is an empty file, with a less than 16 character
49name). The maximum overhead for any non-empty file is the header, and
50the 16 byte padding for the name and the contents, also 16+14+15 = 45
51bytes. This is quite rare however, since most file names are longer
52than 3 bytes, and shorter than 15 bytes.
53
54The layout of the filesystem is the following:
55
56offset content
57
58    +---+---+---+---+
59  0 | - | r | o | m | \
60    +---+---+---+---+ The ASCII representation of those bytes
61  4 | 1 | f | s | - | / (i.e. "-rom1fs-")
62    +---+---+---+---+
63  8 | full size | The number of accessible bytes in this fs.
64    +---+---+---+---+
65 12 | checksum | The checksum of the FIRST 512 BYTES.
66    +---+---+---+---+
67 16 | volume name | The zero terminated name of the volume,
68    : : padded to 16 byte boundary.
69    +---+---+---+---+
70 xx | file |
71    : headers :
72
73Every multi byte value (32 bit words, I'll use the longwords term from
74now on) must be in big endian order.
75
76The first eight bytes identify the filesystem, even for the casual
77inspector. After that, in the 3rd longword, it contains the number of
78bytes accessible from the start of this filesystem. The 4th longword
79is the checksum of the first 512 bytes (or the number of bytes
80accessible, whichever is smaller). The applied algorithm is the same
81as in the AFFS filesystem, namely a simple sum of the longwords
82(assuming bigendian quantities again). For details, please consult
83the source. This algorithm was chosen because although it's not quite
84reliable, it does not require any tables, and it is very simple.
85
86The following bytes are now part of the file system; each file header
87must begin on a 16 byte boundary.
88
89offset content
90
91         +---+---+---+---+
92  0 | next filehdr|X| The offset of the next file header
93    +---+---+---+---+ (zero if no more files)
94  4 | spec.info | Info for directories/hard links/devices
95    +---+---+---+---+
96  8 | size | The size of this file in bytes
97    +---+---+---+---+
98 12 | checksum | Covering the meta data, including the file
99    +---+---+---+---+ name, and padding
100 16 | file name | The zero terminated name of the file,
101    : : padded to 16 byte boundary
102    +---+---+---+---+
103 xx | file data |
104    : :
105
106Since the file headers begin always at a 16 byte boundary, the lowest
1074 bits would be always zero in the next filehdr pointer. These four
108bits are used for the mode information. Bits 0..2 specify the type of
109the file; while bit 4 shows if the file is executable or not. The
110permissions are assumed to be world readable, if this bit is not set,
111and world executable if it is; except the character and block devices,
112they are never accessible for other than owner. The owner of every
113file is user and group 0, this should never be a problem for the
114intended use. The mapping of the 8 possible values to file types is
115the following:
116
117      mapping spec.info means
118 0 hard link link destination [file header]
119 1 directory first file's header
120 2 regular file unused, must be zero [MBZ]
121 3 symbolic link unused, MBZ (file data is the link content)
122 4 block device 16/16 bits major/minor number
123 5 char device - " -
124 6 socket unused, MBZ
125 7 fifo unused, MBZ
126
127Note that hard links are specifically marked in this filesystem, but
128they will behave as you can expect (i.e. share the inode number).
129Note also that it is your responsibility to not create hard link
130loops, and creating all the . and .. links for directories. This is
131normally done correctly by the genromfs program. Please refrain from
132using the executable bits for special purposes on the socket and fifo
133special files, they may have other uses in the future. Additionally,
134please remember that only regular files, and symlinks are supposed to
135have a nonzero size field; they contain the number of bytes available
136directly after the (padded) file name.
137
138Another thing to note is that romfs works on file headers and data
139aligned to 16 byte boundaries, but most hardware devices and the block
140device drivers are unable to cope with smaller than block-sized data.
141To overcome this limitation, the whole size of the file system must be
142padded to an 1024 byte boundary.
143
144If you have any problems or suggestions concerning this file system,
145please contact me. However, think twice before wanting me to add
146features and code, because the primary and most important advantage of
147this file system is the small code. On the other hand, don't be
148alarmed, I'm not getting that much romfs related mail. Now I can
149understand why Avery wrote poems in the ARCnet docs to get some more
150feedback. :)
151
152romfs has also a mailing list, and to date, it hasn't received any
153traffic, so you are welcome to join it to discuss your ideas. :)
154
155It's run by ezmlm, so you can subscribe to it by sending a message
156to romfs-subscribe@shadow.banki.hu, the content is irrelevant.
157
158Pending issues:
159
160- Permissions and owner information are pretty essential features of a
161Un*x like system, but romfs does not provide the full possibilities.
162I have never found this limiting, but others might.
163
164- The file system is read only, so it can be very small, but in case
165one would want to write _anything_ to a file system, he still needs
166a writable file system, thus negating the size advantages. Possible
167solutions: implement write access as a compile-time option, or a new,
168similarly small writable filesystem for RAM disks.
169
170- Since the files are only required to have alignment on a 16 byte
171boundary, it is currently possibly suboptimal to read or execute files
172from the filesystem. It might be resolved by reordering file data to
173have most of it (i.e. except the start and the end) laying at "natural"
174boundaries, thus it would be possible to directly map a big portion of
175the file contents to the mm subsystem.
176
177- Compression might be an useful feature, but memory is quite a
178limiting factor in my eyes.
179
180- Where it is used?
181
182- Does it work on other architectures than intel and motorola?
183
184
185Have fun,
186Janos Farkas <chexum@shadow.banki.hu>
187

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